The applicants have found that in the context of a desktop or document camera, it is strongly desirable to use a strobe or the like to ensure appropriate illumination intensity and uniformity. In such close-up imaging situations, however, specularity caused by the strobe can be a serious problem in the case where the subject has a reflective or glossy surface, as in the case of many magazines, photographs and plastic objects.
In the applicant's copending British Patent Application No. 0103828.0 entitled “Digital Cameras” filed on 16 Feb. 2001, a number of methods are disclosed to ensure that specularity patches are eliminated from the final image. Such methods are generally based on two solutions, namely a single flash solution and a dual flash solution, both of which use at least two initial images to eliminate glare. The results of the disclosed methods is a final image which is devoid of any specular effect, i.e. as if the subject had been illuminated by uniform and diffuse light.
However, it is not always desirable to eliminate glare from a captured image altogether, as in many cases it has aesthetic significance in the sense that it can make the captured image look more natural and ensure that it appears three-dimensional, although in many cases, the glare created in a captured image masks some of the features of the subject, which is clearly undesirable as it detrimentally affects the quality of the captured image.
There are have been several arrangements proposed in the past in connection with digital/analog photography which are concerned with the adjustment or control of light exposure to the subject. For example, U.S. Pat. No. 4,317,620 describes a control system for controlling the intensity of the flash output of an image capturing device, depending on ambient light conditions, thereby varying the amount of glare appearing in the final image.
Initially, the control system measures the ambient scene light and then utilises the result of that measurement to control the intensity or amount of artificial illumination provided during the exposure interval of the image capturing device. If the ambient scene light is below a first predetermined level, the control system selects maximum flash intensity; if the ambient scene light is above a second predetermined level greater than the first predetermined level, the control system selects minimum flash intensity. If the ambient scene light is determined to be between the first and second predetermined levels, then the control system progressively varies the flash intensity in a manner which is inversely proportional to the measured ambient scene light.
However, there is no interaction between the control system and the user of the image capturing device, giving no user control of the specularity or glare appearing in the final image. Further, if the resultant glare is low, this also implies low scene brightness, which is an undesirable effect.
U.S. Pat. No. 4,423,936 describes a photographic exposure control system and method. However, such a system could not be used to attenuate or even remove glare areas within an image because such areas are often saturated so that it is not possible to recover the original image content from the captured pixels.
U.S. Pat. No. 4,541,704 describes a photographic camera apparatus having electronic image enhancement means intended to provide high quality, high resolution photographic prints that have been simultaneously electronically enhanced to compensate either for difficult or unfavourable scene lighting conditions as well as limitations in the film characteristics. The technique used to achieve these aims is often called dual exposure, and involves combining two (or more) images of a subject captured at different (typically increasing) exposure levels. Once again, however, there is no interaction between the user of the image capturing device and the electronic image enhancement means, all so-called enhancement of the final captured image is done automatically without any allowance for user requirements.
U.S. Pat. No. 4,384,336 describes an arrangement which uses algorithmic techniques to selectively enhance segmented areas of a captured image, by comparing the intensities of light being reflected from the subject(s) across the whole field of view of an image capturing device. However, it is not possible to modify glare patches in an image using this type of arrangement, because only a single image is used in the modification method. A single image is insufficient for use in modifying glare patches because such patches are often saturated and, therefore, contain no useful information about the subject. Further, there is little interaction between the user and the system to allow for individual requirements.
There are a few prior art documents which describe dual-flash camera arrangements, such as U.S. Pat. No. 5,345,284 which describes a camera arrangement including an electronic flash unit with two integrated flash heads. However, there is no disclosure or suggestion of the selective or interactive reduction, attenuation or elimination of shadows and/or glare.
International Patent Application No. WO95/32581 describes a system, such as a digitally imaged still photography system which captures a plurality of images simultaneously under different lighting conditions or with different backgrounds, and allows such images to be subsequently blended to exhibit certain characteristics. Similarly, there is a substantial body of prior art which is concerned with the optimisation of a final image using multiple images captured using different settings, including different optical or electrical exposure (e.g. WO00/38417), different magnification and focus settings, different lighting conditions, different spectral properties of the optics, for example colour filters, or a combination of two or more of these.
However, none of the above-mentioned known arrangements are concerned with the attenuation and/or elimination of specularity within a captured image, particularly without detrimentally affecting scene brightness of the final image.
Perhaps the best known method of eliminating specular reflection has been used in the field of photography for many years and uses a single polarizing filter placed in front of the camera lens. By selecting an appropriate angle of rotation of such a filter, it is possible to selectively attenuate/reduce specularity/reflections from the final captured image. A well known variation, often used in machine vision, uses a polarizer to polarize the illumination source and an analyser to filter the captured diffused light only, thereby eliminating the specular component. By rotating the analyser, different amounts of specularity can be selected.